1. Field of the Invention
The invention relates generally to the field of estimating material properties of porous media. More specifically, the invention relates to methods for estimating such properties using computer tomographic (CT) images of porous media such as subsurface rock formation.
2. Background Art
Estimating materials properties such as effective elastic moduli, electrical resistivity and fluid transport properties of porous media, an example of the latter being mobility of hydrocarbon in subsurface rock formations, has substantial economic significance. Methods known in the art for identifying the existence of subsurface hydrocarbon reservoirs, including seismic surveying and well log analysis, need to be supplemented with reliable methods for estimating how fluids disposed in the pore spaces of the reservoir rock formations will flow over time in order to characterize the economic value of such reservoir rock formations.
One method known in the art for estimating fluid transport properties is described in U.S. Pat. No. 6,516,080 issued to Nur. The method described in the Nur patent includes preparing a “thin section” from a specimen of rock formation. The preparation typically includes filling the pore spaces with a dyed epoxy resin. A color micrograph of the section is digitized and converted to an n-ary index image, for example a binary index image. Statistical functions are derived from the two-dimensional image and such functions are used to generate three-dimensional representations of the rock formation. Boundaries can be unconditional or conditioned to the two-dimensional n-ary index image. Desired physical property values are estimated by performing numerical simulations on the three-dimensional representations. For example, permeability is estimated by using a Lattice-Boltzmann flow simulation. Typically, multiple, equiprobable three-dimensional representations are generated for each n-ary index image, and the multiple estimated physical property values are averaged to provide a result.
It is also known in the art to use x-ray computer tomographic (CT) images of samples of rock for analysis. CT images are input to a computer program that segments the images into rock grains and pore spaces. The segmented image can be used as input to programs such as the Lattice-Boltzmann program described above to estimate formation fluid transport properties.
Wellbores drilled through subsurface formations typically have a pipe or casing cemented in place after drilling the wellbore is completed. The casing hydraulically isolates and protects the various rock formations and provides mechanical integrity to the wellbore. The wellbore is hydraulically connected to a formation from which fluid is to be withdrawn or injected by a process known as “perforating.” Perforating is typically performed by inserting an assembly of explosive shaped charges into the wellbore and detonating the charges. See, for example, U.S. Pat. No. 5,460,095 issued to Slagle et al. The process of shaped charge perforating creates a tunnel or flow conduit that allows reservoir fluids to enter the wellbore and subsequently flow or be pumped out of the wellbore. However, by creating the perforation tunnels the physical parameters of the rocks surrounding the tunnel are often altered in such a manner as to restrict or reduce flow.
It is known in the art to test the effectiveness and performance of shaped charges. Testing is typically performed by the shaped charge manufacturer using a procedure specified by the American Petroleum Institute, Washington, D.C. (“API”) known as Recommended Practice 43 (“RP43”). In performing RP43, a target material, typically in the shape of a cylinder, is placed proximate the shaped charge undergoing testing. A steel casing segment or plate and a layer of typical casing cement may be disposed between the target material and the shaped charge. The target material is typically a rock formation known as the Berea sandstone. After detonation of the shaped charge, the dimensions of the perforation made in the target are measured, and the fluid transport properties of the target may be measured in a laboratory. Laboratory evaluation of fluid transport properties can be difficult and expensive. Laboratory evaluation of perforated cores can also be highly inaccurate due to the presence of unknown fractures or heterogeneities within the core.
It is desirable to be able to estimate or determine fluid transport properties of perforation test targets without the need for full laboratory evaluation.